A new diversity index.

We introduce here a new index of diversity based on consideration of reasonable propositions that such an index should have in order to represent diversity. The behaviour of the index is compared with that of the Gini-Simpson diversity index, and is found to predict more realistic values of diversity for small communities, in particular when each species is equally represented and for small communities. The index correctly provides a measure of true diversity that is equal to the species richness across all values of species and organism numbers when all species are equally represented, as well as Hill's more stringent 'doubling' criterion when they are not. In addition, a new graphical interpretation is introduced that permits a straightforward visual comparison of pairs of indices across a wide range within a parameter space based on species and organism numbers.

A myocardial cytotoxic process is involved in the cardiac dysfunction of meningococcal septic shock.

OBJECTIVE: Myocardial dysfunction is a characteristic component of meningococcal septic shock and contributes to the persisting high mortality from the disease. Specific treatment of the myocardial failure has been hampered by the lack of understanding of its pathophysiology. We were interested to determine whether myocardial cell death was occurring in the presence of meningococcal septicemia and whether it correlated with the degree of left ventricular dysfunction and disease severity. We therefore investigated the release of cardiac troponin I (cTnI), a sensitive and specific marker of myocardial cell death, and related this to the severity of disease and cardiac dysfunction. DESIGN: Prospective study SETTING: Pediatric intensive care unit SUBJECTS: Patients admitted to the pediatric intensive care unit with a diagnosis of meningococcal septicemia. INTERVENTIONS: Serum concentrations of cTnI were determined at admission to intensive care in 101 children with meningococcal septicemia and serially in 37 children. Changes in cTnI were related to disease severity as measured by the Pediatric Risk of Mortality score and two markers of cardiac dysfunction. MEASUREMENTS AND MAIN RESULTS: Serum concentrations of cTnI were elevated above the range for healthy children in 24% of children with meningococcal septicemia at admission and in 62% of patients within 48 hrs. The peak concentrations occurred between 12 and 36 hrs after admission. There were significant correlations between cTnI levels and disease severity and between cTnI levels and the degree of myocardial depression measured by quantitative transthoracic echocardiography and peak inotrope requirements. CONCLUSIONS: The elevated serum concentrations of cTnI indicate that myocardial cell death is occurring in meningococcal septicemia. The relationship between cTnI and markers of myocardial function suggest that the cell death may have a role in the pathogenesis of myocardial dysfunction in meningococcal septicemia. Elucidation of the mechanism responsible for myocardial injury may lead to the development of therapeutic interventions to prevent or limit this cardiac damage.

Effect of positioning on respiratory synchrony in non-ventilated pre-term infants.

BACKGROUND AND PURPOSE: Body position can play an important role in an infant's recovery from respiratory disease, but few studies have accounted for sleep state which is known to have a direct influence on the control of respiratory muscles as well as on metabolic and circulatory changes. The purpose of this study was to examine the influence of body position on respiratory function in pre-term infants whilst accounting for sleep state. METHOD: Thoraco-abdominal motion was assessed using respiratory inductance plethysmography (RIP) to provide measures of relative rib cage (RC) and abdominal (AB) movement in ten non-ventilated pre-term infants. Continuous measurements of oxygen saturation (SaO2), pulse and heart rate (HR), were made and sleep state was recorded using behavioural criteria and electro-oculogram (EOG) measurements. RESULTS: The results showed a significant increase in HR in supine, but no significant difference in SaO2 as a function of position, compared to the prone position where a significant reduction was found in thoraco-abdominal asynchrony for both groups and a reduction in variability in both HR and SaO2. Intra-subject variability of thoraco-abdominal motion as a function of position demonstrated no significant difference on return to supine or on return to prone, illustrating good repeatability of measures. CONCLUSIONS: Prone positioning of pre-term infants recovering from respiratory disease may improve respiratory function. As measured, the improvement in respiratory synchrony in prone position brings pre-term infants' breathing pattern into line with that expected in term infants.

Monitoring interactions between spontaneous respiration and mechanical inflations in preterm neonates.

OBJECTIVES: To determine the value of a new bedside monitor in assessing the interactions between spontaneous respiratory activity and ventilator inflations in preterm infants; and to monitor continuously the degree of patient-ventilator synchrony and the stability of spontaneous respiratory effort during different modes of ventilation and in response to care procedures. DESIGN: A prospective, observational study of physiologic variables recorded by a computerized monitoring system. SETTING: A neonatal intensive care unit in a teaching hospital. PATIENTS: Thirty-one neonates (median gestational age of 28 wks [range 24 to 36]; median birth weight of 942 g [range 624 to 2940]) were monitored during conventional mandatory ventilation at rates ranging from 47 to 108 inflations/min, and 22 infants (median gestational age of 27.5 wks [range 25 to 40]; median birth weight of 1345 g [range 510 to 3490]) were monitored during patient-triggered ventilation. All infants were sedated as part of the routine care policy. INTERVENTIONS: Spontaneous respiration (abdominal pressure capsule) and ventilator inflations (airway pressure) were recorded continuously for periods of up to 3 days in mechanically ventilated preterm infants. MEASUREMENTS AND MAIN RESULTS: The monitoring system uses the Frequency Tracking Locus method to derive the interaction Score, which quantifies the degree of entrainment of the spontaneous respiratory pattern by the ventilator. This analysis was applied to airway pressure and abdominal capsule signals. A perfect 1:1 interaction between spontaneous inspirations and mechanical inflations returns an interaction Score of 1.00, and irregular interactions return a score of > 1.5. During conventional mandatory ventilation, a total of 53,074 16-sec epochs (representing 782,811 spontaneous breaths) were studied in 31 preterm infants: 27.4% of epochs showed a 1:1 interaction, 60.5% a non 1:1 interaction, and 12.1% indicated a passive (i.e., infant apneic) response by the infant, despite excluding periods when paralyzing agents were used. The median interaction Score value during 1:1 interactions was 1.2, whereas for non 1:1 interactions the interaction Score was 2.2. One to one entrainment occurred at conventional mandatory ventilation rates between 50 and 85 inflations/min: for many infants, such entrainment was achievable over a range of conventional mandatory ventilation rates, while in some infants respiration was unstable at all rates of conventional mandatory ventilation. During passive ventilation, the median Interaction Score was 1.0. During patient-triggered mechanical ventilation, approximately 67,150 spontaneous respiratory cycles, represented by 3,592 16-sec epochs, were studied in 22 infants. Overall, 19.5% (702) of epochs showed the criteria for ideal triggering by spontaneous inspiration and 19.6% (703) showed autotriggering. In 60.9% (2187) of epochs, a non 1:1 interaction was noted. During ideal patient-triggered mechanical ventilation, the median interaction Score was 1.14; during passive (autotriggered) ventilation, the median Interaction Score was 1.05; and during non 1:1 ventilation, the median score was 1.74. "Autotriggering" was found frequently in infants of < or = 28 wks gestation. The monitor was able to distinguish between stable and unstable interactions and apnea during conventional mandatory ventilation and patient-triggered mechanical ventilation by reference to the Interaction Score value. CONCLUSIONS: We describe a new kind of bedside monitor for the Interpretation of respiratory data. Unlike other methods, it is able to give the clinician a continuous measure of patient-ventilator interaction which is easy to interpret. It appears to have wide-spread application in neonatal intensive care nurseries where the babies' own breathing efforts can affect the efficiency of respiration and cause unwanted physiologic instability. The monitor can be used to determine the optimal ventilatory settings to

Effects of intermittent mandatory ventilation on respiratory timing in preterm infants.

Twenty preterm infants (25-36 weeks' gestation) were studied during intermittent mandatory ventilation (IMV) at rates of 10-34 inflations/min. Airway pressure and abdominal capsule signals were recorded at varying postnatal ages. Spontaneous interbreath interval (IBI), inspiratory (Ti) and expiratory (Te) duration were measured over 100-1000 ventilator cycles. Baseline Te was 0.48 s (+/- 0.129) and increased to 0.65 s (+/- 0.182) when associated with a mechanical inflation. Baseline Ti was 0.34 s (+/- 0.062) and increased to 0.38 s (+/- 0.081) with inflation. IBI increased from 0.82 s (+/- 0.161) to 1.03 s (+/- 0.201) with inflation. 1:1 entrainment (phase-locking) was observed at rates of ventilation below the spontaneous respiratory rate but spontaneous inspiration and mechanical inflation were always out of phase. A linear relationship was noted between the prolongation of IBI and the timing of inflation within the spontaneous respiratory cycle during IMV. This relationship could be used to estimate the range of rates of mechanical inflation capable of inducing 1:1 entrainment.

The variation with gestational age of the rheological properties of the blood of the new-born.

A study has been made of the variation of blood viscosity and related factors with the gestational age of neonates from 24 weeks to normal term. Viscosity increases significantly over this period by 36% at high shear rate (128.5 s-1) and 250% at low shear (0.277 s-1). The high shear rate changes can be explained largely by the effects of variations in haematocrit and plasma viscosity. At low shear rate the same factors are involved together with changes in the plasma protein composition, in particular the age-related increase in the concentration of the proteins known to induce rouleaux formation. The variation in the degree of sialination of fibrinogen with gestational age may also play a part.

Use of the frequency-tracking locus in estimating the degree of respiratory entrainment in preterm infants.

In order to define the complex interactions between external stimuli and non-linear physiological systems, a technique (the frequency-tracking locus, FTL) was devised that describes the cycle-by-cycle changes in phase angle and amplitude between two signals. Qualitative assessment of the nature of interactions between the signals can be made by examining the FTL. Quantitation of the extent of entrainment of the spontaneous physiological rhythm is possible after deriving a numerical index (the path-length index, PLI) describing the departure of the system from a fully entrained state. The FTL was applied to the study of interactions between spontaneous respiratory effort and mechanical inflation in preterm newborn babies undergoing mechanical ventilation. Stable and unstable states of 1:1 interaction were noted while integer-ratio relationships were seen at low rates of mechanical ventilation. Stable states of entrainment corresponded to a PLI value near unity, and the value of PLI increased rapidly as interactions became unstable. The FTL may be used to describe complex interactions in physiological systems, and may be used as a guide to baby-ventilator matching during mechanical ventilation of the newborn.

The cardiovascular and oncotic effects of albumin infusion in premature infants.

We determined the oncotic and cardiovascular effects of a standardised infusion of human albumin (1.2 g/kg over 2 h as a 20% solution) in 12 premature infants on 18 occasions when hypovolaemia was suspected on clinical grounds. Blood volume increased by a median value of 15.5%, and fell to preinfusion values by 3 h post infusion in all but four cases. Albumin concentration and colloid osmotic pressure rose during infusion and remained raised even when blood volume had fallen to preinfusion levels. Blood pressure rose in 3 cases only and heart rate fell by greater than 5 beats/min in 6 cases. Indices of long- and short-term heart rate variability were unchanged, but blood pressure variability fell in the second hour of infusion (P = 0.03), an effect which was independent of changes in lung inflation. No changes in blood gases or oxygenation occurred during infusion and no evidence of pulmonary oedema was found. There were wide variations in oncotic and cardiovascular responses to the standardised infusion both between and within subjects. When human albumin is infused in this manner some protection against respiration-induced variability in blood pressure can result, but the circulatory response may prove difficult to predict in the individual.

A comparison of the rheologic properties of neonatal and adult blood.

A number of studies have indicated that the rheologic properties of neonatal blood are different from those of the adult. The frequent administration of blood components to the neonate during intensive care make it important that these differences be established and their causes understood. The purpose of this study was to make a detailed comparison of the rheologic properties of neonatal and adult blood, with particular emphasis on low shear rate viscosity and rouleaux-related phenomena. The viscometric data was obtained from seven preterm (PT) and 18 normal term (NT) babies and compared with those from 18 adults (A). In the present study, viscometry was performed over a wide range of shear rates, from about 0.3 to 130 s-1, and the low shear rate data were compared with direct measurement of rouleaux formation using the Myrenne Erythrocyte Aggregometer. A major factor leading to the viscometric differences observed was the high hematocrit common in the newborn (46.8 +/- 2.1% PT, 52.8 +/- 6.1% NT, 44.1 +/- 2.5% A males, 40.5 +/- 1.9% A females). However, this tended to be compensated for by the lower plasma viscosity (1.05 +/- 0.07 mPas PT, 1.23 +/- 0.14 mPas NT, 1.34 +/- 0.08 mPas A--no sex difference) and reduced rouleaux formation observed in the newborn and more marked in the preterm baby. The lowered levels of red cell aggregation were found not to be due to cellular differences between the adults and the babies but rather to differing plasma components. The presence of the fetal variant of fibrinogen and low levels of immunoglobulins, especially IgM and IgA, are likely to be of particular importance.

Quantification of cardiovascular instability in premature infants using spectral analysis of waveforms.

Spectral analysis was applied to blood pressure and cerebral blood flow velocity recordings in premature infants with respiratory distress in order to quantify respiration-induced cardiovascular variability. Aortic blood pressure was transduced via an umbilical arterial catheter and cerebral blood flow velocity measured in the anterior cerebral artery using a 10 MHz continuous wave Doppler velocimeter in 16 infants less than or equal to 32 wk gestational age. Spectral analysis of the resulting waveforms revealed heart rate and respiratory rate components whose relative amplitudes (heart rate/respiratory rate amplitude ratio) represent an index of that component of variability induced by respiratory events. The mean (heart rate/respiratory rate amplitude) ratio was 47.2 in spontaneously breathing infants and rose to 165.9 in infants who were ventilated during muscle paralysis (p = 0.0003). Cerebral blood flow velocity recordings showed R components in only 22 of 38 simultaneous recordings. This method can be used to quantify respiration-induced cardiovascular variability and its response to therapy, and may provide a means of identifying infants at risk from brain injury due to an inability to regulate cerebral blood flow.

Pulse oximeter and transcutaneous arterial oxygen measurements in neonatal and paediatric intensive care.

Pulse oximeter (SaO2P) measurements were compared with direct arterial line oxygen saturation (SaO2) from co-oximeters in 92 instances in 43 patients, and with arterial line oxygen measurements (PaO2) in 169 instances in 81 patients. The mean (SD) absolute difference between SaO2P and SaO2 was 2.6% (2.4) after attempt to correct for the co-oximeter falsely measuring a proportion of fetal haemoglobin as carboxy haemoglobin. For 19 infants and children greater than or equal to 5 months old, who have very little fetal haemoglobin, the mean (SD) absolute difference of 27 comparisons was 1.8% (2.1). Comparison of SaO2P and PaO2 measurements in 46 instances when PaO2 was less than 6.67 kPa showed SaO2 to be less than 90% on 40 occasions. In 24 instances when PaO2 was greater than or equal to 13.3 kPa the SaO2P was greater than or equal to 98% on 22 occasions. In 23 infants undergoing neonatal intensive care, transcutaneous oxygen monitors were compared with arterial PO2 measurements in 60 instances. The mean (SD) absolute difference between PaO2 and transcutaneous oxygen measurements was 1.60 kPa (1.73). Ten of the 60 comparisons had differences greater than 2.67 kPa and three greater than 5.33 kPa (maximum 8.40 kPa). Pulse oximetry is a clinically useful technique for managing oxygenation but further studies are needed to confirm its safety in premature infants at risk of retinopathy of prematurity.